Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric inter-layers and conductive paths or interconnects made of conductive materials. Copper and copper alloys have recently received considerable attention as interconnect materials because of their superior electromigration and low resistivity characteristics. The interconnects are usually formed by filling copper by a metallization process in features or cavities etched into the dielectric inter-layers. The preferred method of copper metallization is electroplating. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in sequential interlayers can be electrically connected using vias or contacts.
In a typical interconnect fabrication process, first, an insulating interlayer is formed on the semiconductor substrate. Patterning and etching processes are performed to form features such as trenches and vias in the insulating layer. Then, copper is electroplated to fill all the features after a conductive barrier and a seed layer are deposited. The plating process results in a thick copper layer on the substrate some of which need to be removed before the subsequent step. Conventionally, after the copper plating, CMP process is employed to globally planarize and then reduce the thickness of this excess copper overburden down to the level of the surface of the barrier layer, which is then also removed, leaving conductors only in the features. However, CMP process is a costly and time-consuming process that needs to be reduced.
The adverse effects of conventional material removal technologies such as CMP may be minimized or overcome by employing an Electrochemical Mechanical Processing (ECMPR) approach for conductor deposition. ECMPR has the ability to provide thin layers of planar conductive material on the workpiece surface, or even provide a workpiece surface with no or little excess conductive material. The term of Electrochemical Mechanical Processing (ECMPR) is used to include both Electrochemical Mechanical Deposition (ECMD) processes as well as Electrochemical Mechanical Etching (ECME), which is also called Electrochemical Mechanical Polishing (ECMP). It should be noted that in general both ECMD and ECME processes are referred to as ECMPR since both involve electrochemical processes and mechanical action.
FIG. 1 shows an exemplary conventional ECMPR system 10, which includes a workpiece-surface-influencing device (WSID) 12 such as a mask, pad or a sweeper, a carrier head 14 holding a workpiece 15 and an electrode 16. Other conventional ECMPR systems include those that have reverse geometry, i.e. WSID is above the workpiece and the workpiece surface faces up.
During ECMD or ECME processes, the WSID 12 may be held in close proximity of the workpiece surface, i.e., no-touch processing, or may mechanically sweep the surface of the workpiece, i.e. touch-processing, while a relative motion is established between the workpiece surface and the WSID.
Surface of the WSID 12 mechanically sweeps the surface of the workpiece 15 while an electrical potential is established, at least during a portion of the total process time, between the electrode 16 and the surface of the workpiece during touch-processing. Channels 18 of the WSID 12 allow a process solution 20 such as an electrolyte to flow to the surface of the workpiece 15. If the ECMD process is carried out, the surface of the workpiece 15 is wetted by a deposition electrolyte, which is also in fluid contact with the electrode 16 and a potential is applied between the surface of the workpiece and the electrode rendering the workpiece surface cathodic. If the ECME process is carried, out, the surface of the workpiece is wetted by the deposition electrolyte or a special etching or electroetching or polishing liquid, which is also in fluid contact with an electrode and a potential is applied between the surface of the workpiece and the electrode rendering the workpiece surface anodic. Thus, etching takes place on the workpiece surface. Very thin planar deposits can be obtained by first depositing a planar layer using an ECMD technique and then using an ECME technique on the planar film in the same electrolyte by reversing the applied voltage. Alternately, the ECME step can be carried out in a separate machine and a different etching electrolyte. This way the thickness of the deposit may be reduced in a planar manner.
Descriptions of various planar deposition and planar etching methods, i.e. ECMPR approaches and apparatus can be found in the following patents and pending applications, all commonly owned by the assignee of the invention: U.S. Pat. No. 6,126,992 entitled “Method and Apparatus for Electrochemical Mechanical Deposition,” U.S. application Ser. No. 09/740,701 entitled “Plating Method and Apparatus that Creates a Differential Between Additive Disposed on a Top Surface and a Cavity Surface of a Workpiece Using an External Influence,” filed on Dec. 18, 2001, and U.S. application Ser. No. filed on Sep. 20, 2001 with Ser. No. 09/961,193 entitled “Plating Method and Apparatus for Controlling Deposition on Predetermined Portions of a Workpiece”. These methods can deposit metals in and over cavity sections on a workpiece in a planar manner. They also have the capability of yielding novel structures with excess amount of metals selectively over the features irrespective of their size, if desired.
The surface of the WSID preferably contains a hard and abrasive material for efficient sweeping, although softer materials may also be used if high planarization efficiency is not necessary for the specific application or the workpiece surface contains materials that are structurally weak. U.S. application with Ser. No. 09/960,236 filed on Sep. 20, 2001, entitled “Mask Plate Design,” and U.S. Utility Application filed on May 23, 2002 entitled “Low Force Electrochemical Mechanical Processing Method and Apparatus,” that claims priority from application Ser. No. 60/326,087 filed on Sep. 28, 2001, all assigned to the same assignee as the present invention, disclose various workpiece-surface-influencing device embodiments. Fixed abrasive sheets or pads, which are supplied by companies such as 3M and which are commonly used in CMP applications, work efficiently also on WSID surfaces for ECMPR applications. As exemplified in FIG. 2, such abrasive sheets 30 generally comprise abrasive composites 32 that have a discernible precise shape such as pyramidal or cylindrical. The abrasive composite shapes include a plurality of abrasive grains 34 dispersed in a binder 36. The abrasive composite is bonded to a backing layer 38 through some bonding agent or film (not shown).
During a CMP process, the top surface 33 of the abrasive sheet is used to abrade and polish a workpiece surface by pressing the workpiece surface onto the top surface 33 or vice versa. As polishing action continues, the abrasive composite shapes break down slowly and expose unused abrasive grains embedded in the binder. Thus, the height “H” of composite shape shown in FIG. 2 gets smaller and smaller. As the sheet is used for an extended time, the composite shapes further break down and expose more fresh abrasive grains. Eventually, when the height H of the composite shapes is close to zero, the sheet is replaced. As the brief review above demonstrates the so-called “fixed-abrasive pads” described above actually do not permanently fixed within a matrix. In fact, they are designed to slowly break down, exposing a new surface with fresh abrasive particles.
During the process, the wafer surface makes contact with the abrasive particles as well as the matrix material surrounding them. The matrix material is generally a polymeric material. This design of a composite abrasive film is attractive for CMP applications where the polishing products, including the breakdown products of the abrasive film are washed off the surface of the wafer along with the material removed from the surface of the wafer. For an ECMPR process, however, the surface of the WSID touches the wafer surface during film deposition, before all the features or cavities on the wafer surface are filled with the conductor. Due to the constant breaking down of the composite abrasive layer, such abrasive sheets have relatively short lifetime and need to be replaced often, e.g. every 100-1000 wafers. Also constant shedding of minute abrasive grains, matrix or binder pieces into the process solutions may pollute the process solution, and may even get included in the deposited layers, which are undesirable situations.
For ECME applications, similarly, particle shedding by the pads is undesirable. Other materials that may be used on the surface of the WSID structures that make physical contact to the wafer surface during ECMPR include polymeric pads that are commonly used in CMP applications. These polymeric pads are supplied by companies such as Rodel and Thomas West. The polymeric materials, however, are not very durable in electrolytes of ECMPR. They wear in time and shed microscopic polymeric particles, which may in turn get into the copper film that is deposited. Such drawbacks lower throughput of ECMPR process, increase cost of consumables and also adversely affect product consistency.
Therefore, it will be desirable to provide a longer life WSID surface that does not shed harmful particles for ECMPR applications such as ECMD and ECME.